Battery over-current protection circuit

ABSTRACT

A first series circuit of a current detector and a first switch is connected in series with a battery, and a second series circuit of a resistor and a second switch is connected in parallel with the first series circuit. A control device controls an on and off state of the first and second switches to provide over-current protection of the battery. In particular, the control device turns off the first switch to cut-off battery over-current and turns on the second switch when the battery current exceeds a specified level as detected by the current detector. Then, after the battery current, which flows through the second series current, drops below the specified level, the first switch is returned to an on state to again allow battery discharge.

BACKGROUND OF THE INVENTION

This invention relates to a circuit to protect batteries fromover-current. When a non-rechargeable battery, which can only bedischarged, or a rechargeable battery, which can be charged ordischarged, is discharged (or charged for the case of a rechargeablebattery) with a current in excess of that specified ("over-current"),the battery heats up, performance drops, and life-time is degraded.

As recited in Japanese Non-examined Patent Publication No. 63-158744published Jul. 11, 1988, over-current protection circuits utilizingbreakers and PTC devices (devices which increase resistance due to Jouleheating) have been developed to prevent over-current discharge ofbatteries.

Unfortunately, an over-current protection circuit using breakers or PTCdevices cannot prevent over-current discharge in a desirable manner. Forexample, in an over-current protection circuit using a PTC device, sometime is required for the resistance of the PTC device to increase due toover-current. Therefore, during the time required for the resistance ofthe PTC device to increase, the battery has already discharged withexcessive current. It is thus the first object of the present inventionto overcome this drawback and protect a battery against over-currentdischarge and/or charge.

An over-current protection circuit, provided with elements such assemiconductor switches for quickly interrupting discharge whenover-current is detected, can cut-off a switch as soon as batterydischarge current above the specified value is detected.

This type of over-current protection circuit can immediately cut-off aswitch to reliably suppress battery heating due to over-current.However, this type of circuit has the following problem. Namely, whenthe battery is connected to electrical equipment and the electricalequipment power switch is turned on, a starting surge of in-rush currentgreater than that for normal operation flows for an instant.Consequently, an over-current protection circuit, that immediately cutsoff battery discharge when a current over the specified value isdetected, will also cut-off due to starting surge. An over-currentprotection circuit with the over-current specification increased toavoid cut-off at the starting surge cannot safely protect the batteryfrom over-current discharge. It is thus the second object of the presentinvention to provide an over-current protection circuit that canappropriately protect a battery from over-current without faultyoperation caused by large starting surge currents.

An over-current protection circuit with semiconductor switches toquickly cut-off discharge when over-current is detected, can immediatelycut-off a switch to reliably suppress battery heating when batterydischarge current is detected above the specified value. For example,over-current discharge can be effectively prevented even in the case ofa momentary short circuit. However, when the momentary battery shortcircuit and over-current condition is relieved, it is necessary for thecut-off switch to be returned to a condition allowing suitabledischarge. It is thus the third object of the present invention toprovide an over-current protection circuit that can reliably preventbattery over-current, as well as immediately return to a state allowingdischarge after the over-current condition has been relieved.

SUMMARY OF THE INVENTION

The battery over-current protection circuit of this invention comprisesa battery, a current detection means and first switching means connectedin series with the battery, a resistor and second switching meansconnected in parallel with the current detection means and firstswitching means, and a control means to turn the first and secondswitching means on and off.

When battery current exceeds the specified current, the control meansturns the first switching means off and the second switching means on.When battery current drops below the specified current, the controlmeans turns the first switching means on.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a battery over-current protectioncircuit of the present invention.

FIG. 2 is a simplified circuit diagram showing operation state in whichthe switch 5 is closed and the switch 4 is open in the circuit of FIG.1.

FIGS. 3-9 respectively show examples of the battery over-currentprotection circuit of the present invention in which a MOSFET isemployed as a switch device and a comparator is employed as a switchcontrol device.

FIG. 10 is a wave-form graph explaining the operation of the circuitshown in FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

Over-current charge and/or discharge is prevented by the over-currentprotection circuit of the present invention by turning the firstswitching means off and the second switching means on to reduce batterycurrent with a resistor when that current exceeds the specified value.When battery current drops below the specified value, the firstswitching means is turned on for normal charge and/or discharge.

FIG. 1 is a circuit diagram of an over-current protection circuit forthe first embodiment of the present invention applied to the case of arechargeable battery. In this figure, 1 is a rechargeable battery suchas a lithium ion rechargeable battery, 2 is a current detection meansconnected in series with the rechargeable battery 1 and comprising a lowvalued (for example, 100 mΩ) resistor R1 and a first voltage detector 3to measure the voltage across R1, 4 is the first switching meansconnected in series with resistor R1, 5 is the second switching meansconnected in parallel with the series connection of resistor R1 and thefirst switching means 4, and R2 is a high valued (for example, 100 KΩ)second resistor connected in series with the second switching means 5and in parallel with the series combination of resistor R1 and the firstswitching means 4.

The first and second switching means 4, 5 can be Metal OxideSemiconductor Field Effect Transistors (MOSFET's). In this case, theinternal resistance of the MOSFET can also be used as resistor R1. Thereis no need to provide a separate resistor R1 in a circuit using theinternal resistance of the MOSFET as R1, and the first voltage detector3 is connected to measure the voltage across the MOSFET terminals (drainand source).

Finally, 6 are discharge terminals comprising the positive terminal ofthe rechargeable battery 1 and one terminal of the first switching means4, 7 is a second voltage detector to measure the voltage across thedischarge terminals 6, 8 is a switch control means to turn the first andsecond switching means 4, 5 on and off based on measurements from thefirst and second voltage detectors 3, 7, and 9 is the load connected tothe discharge terminals 6 of the rechargeable battery 1.

This over-current protection circuit has the load 9 connected to itsdischarge terminals 6. When the rechargeable battery 1 is beingdischarged, the first switching means 4 is turned on and the secondswitching means 5 is turned off. The discharge current of therechargeable battery 1 is monitored by the first voltage detector 3measuring voltage across resistor R1.

Now if a low resistance load 9 (or a short circuit) is connected to thedischarge terminals 6 and if the discharge current of the rechargeablebattery 1 exceeds 5 A, the first voltage detector 3 will detect that. Inother words if the voltage across the terminals of R1 exceeds 0.5 V, thefirst voltage detector 3 outputs a detection signal. This detectionsignal is input to the switch control means 8. Based on the detectionsignal, the switch control means 8 turns the first switching means 4 offand the second switching means 5 on. By doing this, the dischargecurrent of the rechargeable battery 1 is limited to an extremely smallvalue by resistor R2, and the battery 1 is protected againstover-current.

Turning to FIG. 2, the simplified circuit diagram makes this situationeasily understood. Since the resistance of the load 9 is much smallerthan that of resistor R2, the second voltage detector 7 measuresapproximately 0 V.

If the resistance of the load 9 increases, the second voltage detector 7measures a voltage corresponding to the value of the load 9 resistance.In particular, if the load 9 is removed (load resistance becomesinfinite), the second voltage detector 7 measures the rechargeablebattery voltage. Therefore, when the second voltage detector 7 measuresa voltage above a specified value (corresponding to the specified valueof rechargeable battery current), the switch control means 8 turns thefirst switching means 4 on and the second switching means 5 off. Indoing this, the rechargeable battery 1 returns to a state capable ofnormal discharge.

It is emphasized that the over-current protection circuit of the presentinvention is not limited to discharge of a rechargeable battery asdescribed above, but is also effective as a protection circuit duringrechargeable battery charging as well as during non-rechargeable batterydischarge.

In an over-current protection circuit with the configuration describedabove, the first switching means is turned off and the second switchingmeans is turned on by the control means when battery current exceeds thespecified value. When battery current drops below the specified value,the first switching means is returned to the on state. This type ofcircuit is characterized in that the battery is reliably protectedagainst over-current during battery discharge and/or charge.

Turning to FIG. 3, an over-current protection circuit, that cuts off aswitch when over-current flows longer than a set time interval, isshown. This over-current protection circuit does not cut-off the switchfor in-rush current that flows only for a brief interval.

FIG. 3 is a circuit diagram of the second embodiment of the presentinvention. In this figure, 31 is the battery which is either anon-rechargeable battery such as a dry cell or a rechargeable batterysuch as a lithium ion rechargeable battery, 32 is a current detectionmeans which is low value resistor connected in series with the battery31 to convert current into voltage, 33 is a switching means which is aMOSFET connected in series with the current detection means 32, 34 is adelay circuit which is comprised of a resistor 35 and capacitor 36connected in parallel with the current detection means 32, and 37 is acomparator which compares the voltage to which capacitor 36 has beencharged with a reference voltage E0 and turns off switching means 33 ifthe capacitor voltage becomes greater. Together the delay circuit 34 andthe comparitor 37 make up the control means.

In the case of battery 31 discharge in this over-current protectioncircuit, discharge current flows through the current detection means 32and the delay circuit 34. For normal discharge current, the capacitor 36charges to a voltage which does not exceed the reference voltage E0.Consequently, the comparator output is a high level signal whichmaintains the switching means 33 in the on state.

When over-current flows in the battery 31, the capacitor 36 is graduallycharged up, and after the set time interval its voltage exceeds thereference voltage E0. This causes the comparator 37 to output a lowlevel signal which turns off the switching means 33 and cuts off battery31 discharge. When over-current flows in the battery 31 for only a briefinterval, the capacitor 36 is not charged to a voltage that exceeds thereference voltage E0. Consequently, the switching means 33 is not turnedoff.

FIG. 4 is a circuit diagram of the third embodiment of the presentinvention. In this figure, 410 is the first comparator which comparesthe voltage detected by the current detection means 42 with referencevoltage E1, 411 is a current source, and 412 is a MOSFET. The seriescombination of current source 411 and MOSFET 412 is connected inparallel with the battery 41. The MOSFET 412 is controlled by the outputof the first comparator 410. Continuing, 413 is a capacitor connected inparallel with MOSFET 412, and 414 is the second comparator whichcompares the voltage across capacitor 413 with reference voltage E2 andcontrols the switching means 43 based on the results.

When normal discharge current flows in the battery 41 of thisover-current protection circuit, the output of the first comparator 410is a high level signal causing the MOSFET 412 to be in the on statepreventing the capacitor 413 from charging. Consequently, the output ofthe second comparator 414 is a high level signal that maintains theswitching means 43 in the on state.

When over-current flows in the battery 41, the output of the firstcomparator 410 becomes a low level signal turning the MOSFET 412 off.This causes constant current charging of the capacitor 413 by thecurrent source 411, and after the set time interval the capacitorvoltage exceeds reference voltage E2. When this occurs, the output ofthe second comparator 414 becomes a low level signal, the switchingmeans 43 is turned off, and battery 41 discharge is cut-off.

When over-current does not flow through the battery 41 for a periodlonger than the set time interval, the MOSFET 412 switches to the onstate before the capacitor 413 voltage builds up to the referencevoltage E2. The capacitor 413 then discharges while the secondcomparator 414 maintains a high level output signal without everchanging to a low level signal. Consequently, the switching means 43 isnever turned off.

FIG. 5 is a circuit diagram of the fourth embodiment of the presentinvention. In this figure, 520 is a differential amplifier whichmeasures the voltage across the terminals of the current detection means52 resistor, and 521 is a voltage controlled current source whichproduces a fixed current in proportion to the output of the differentialamplifier 520 and is connected in parallel with current source 511.Those circuit elements that are the same as the third embodiment of FIG.4 have the same identification number except for the left-most digit.

When normal discharge current flows in the battery 51 of thisover-current protection circuit, the output of the first comparator 510is a high level signal causing MOSFET 512 to be in the on statepreventing capacitor 513 from charging. Consequently, the output of thesecond comparator 514 is a high level signal that maintains theswitching means 53 in the on state.

When over-current flows in the battery 51, the output of the firstcomparator 510 becomes a low level signal turning the MOSFET 512 off.Consequently, the capacitor 513 is charged with constant current by thecurrent source 511 and the voltage controlled current source 521. Atthis time, the voltage controlled current source 521 produces a currentproportional to the output voltage of the differential amplifier 520. Inother words, the larger the battery 51 current, the larger the currentsource 521 current. Therefore, the time interval required for capacitor513 to charge up to a voltage exceeding reference voltage E2 is shorterthe larger battery current.

When the capacitor 513 charges to a voltage greater than the referencevoltage E2, the second comparator 514 output becomes a low level signal,the switching means 53 gets turned off, and battery 51 discharge iscut-off.

In other words, in this embodiment, the greater the battery 51 current,the faster the capacitor 513 is charged and the shorter the timeinterval before the switching means is turned off.

When over-current flows through the battery 51 but only for a briefinterval, the MOSFET 512 switches to the on state discharging capacitor513 before its voltage builds up to the reference voltage E2.Consequently, the second comparator 514 maintains a high level outputsignal and the switching means 53 is never turned off.

FIG. 6 is a circuit diagram of the fifth embodiment of the presentinvention. In this figure, 630 is the first comparator which comparesthe measured value from the current detection means 62 with the firstreference voltage E3, 631 is the second comparator which compares themeasured value from the current detection means 62 with the secondreference voltage E4, 632 is the first MOSFET, 633 is the first currentsource, and 634 is the second MOSFET. The first MOSFET 632, the firstcurrent source 633, and the second MOSFET 634 are connected in series inthat order and their combination is connected in parallel with thebattery 61. The first MOSFET 632 is a normally off (enhancement mode)FET controlled by the output of the first comparator 630, and the secondMOSFET 634 is a normally on (depletion mode) FET controlled by theoutput of the second comparator 631. Continuing, 635 is the secondcurrent source connected in parallel with the series combination of thefirst current source 633 and the second MOSFET 634, 636 is a capacitorconnected in parallel with the first MOSFET 632, and 637 is the thirdcomparator which compares the voltage across capacitor 636 with thethird reference voltage E5 and controls the switching means 63 accordingto the results.

In this over-current protection circuit, the first reference voltage E3is related to the second reference voltage E4 such that E3<E4. In thefollowing explanation, the first comparator 630 outputs a low levelsignal when a current of 5 A or more flows, and the second comparator631 outputs a low level signal when a current of 15 A or more flows.

When normal discharge current (namely, less than 5 A) flows in thebattery 61 of this over-current protection circuit, the outputs of boththe first comparator 630 and the second comparator 631 are high levelsignals causing the first MOSFET 632 to be in the on state (and thesecond MOSFET 634 to be in the off state) preventing capacitor 636 fromcharging. Consequently, the output of the third comparator 637 is a highlevel signal that maintains the switching means 53 in the on state.

When more than 5 A but less than 15 A of over-current flows in thebattery 61, the output of the first comparator 630 goes low and thefirst MOSFET 632 turns off. On the other hand, the output of the secondcomparator 631 remains high and the second MOSFET 634 remains in the offstate. Consequently, capacitor 636 is charged by the second currentsource 635, and after the set time interval, the voltage acrosscapacitor 636 exceeds the third reference voltage E5. This causes thethird comparator 637 to output a low signal turning the switching means63 off and cutting off discharge from the battery 61.

When more than 15 A of over-current flows in the battery 61, the outputsof both the first and second comparators 630, 631 go low, the firstMOSFET 632 turns off, and the second MOSFET 634 turns on. Consequently,capacitor 636 is charged by both the first and second current sources633, 635, and the time interval until the capacitor 636 voltage exceedsthe third reference voltage E5 is much shorter than when the capacitor636 is charged only by the second current source 635. When E5 isexceeded, the third comparator 637 output drops low, the switching means63 is turned off, and battery 61 discharge is cut-off.

In other words, in this embodiment, depending on the battery 61 current,there are two different set time intervals until the switching meansturns off.

In this embodiment as in the previously described embodiments, whenover-current flows through the battery 61 for an interval shorter thanthe set intervals, the first MOSFET 632 switches to the on statedischarging capacitor 636 before its voltage builds up to the thirdreference voltage E5. Consequently, the third comparator 637 maintains ahigh level output signal and the switching means 63 is never turned off.

In the above embodiments, a resistor is provided as the currentdetection means to measure battery current. However, by measuring thevoltage across the MOSFET used as a switching means, the MOSFET can alsoserve as the current detection means. In which case, a resistor inunnecessary.

In the over-current protection circuits of FIG. 3 through FIG. 6, theswitching means is turned off by the control means when greater than thespecified battery current flows for longer than the set time interval.Therefore, there is no unwanted switch cut-off due to in-rush currentand the battery is appropriately protected against over-current.

In the following over-current protection circuits, when over-current isdetected, the switching means is turned off stopping battery discharge,then after a set time interval, the switching means is automaticallyturned on. If the over-current condition continues after the set timeinterval, the switch is again turned off. If the over-current conditionis corrected after the set time interval, the switch stays on and thebattery returns to a condition capable of discharge.

FIG. 7 is a circuit diagram of the sixth embodiment of the presentinvention. In this figure, 71 is either a rechargeable ornon-rechargeable battery, 72 is a current detection means which is lowvalue resistor connected in series with the battery 71, and 73 is aMOSFET switching means connected in series with the current detectionmeans 72.

Continuing, 74 is the first comparator which compares the voltagemeasured by the current detection means 72 with the reference voltageE1, 75 is a flip-flop (abbreviated FF in the following) which is setbased on the output of the first comparator 74, 76 is a current source,and 77 is a MOSFET. The current source 76 and MOSFET 77 are a seriescombination connected in parallel with the battery 71, and the MOSFET 77is controlled on or off by the output of the FF 75 connected through aninverter 78. Further, 79 is a capacitor connected in parallel withMOSFET 77, and 710 is the second comparator which compares the voltageacross the capacitor 79 with the reference voltage E2 and resets the FF75 depending on the results of comparison. The first comparator 74 andthe second comparator 710 make up the control means to control theswitching means 73 on or off depending on the FF 75 output.

When normal discharge current (less than 5 A) flows in the battery 71 ofthis over-current protection circuit, the output of the first comparator74 is a high level signal. As a result, the FF 75 is in the reset stateand its output is low. Consequently, the MOSFET 77 is on, capacitor 79is not charged, and the output of the second comparator 710 is high.Therefore, the FF 75 is maintained in the reset state and the switchingmeans 73 is never turned off.

When over-current (for example, more than 5 A) flows in the battery 71,the first comparator 74 output drops low and the FF 75 is set. As aresult, the inverter 78 output goes low, MOSFET 77 is turned off, andthe switching means 73 is turned off at the same time cutting offbattery 71 discharge.

When MOSFET 77 is turned off, capacitor 79 begins charging due tocurrent source 76, and after a set time interval (for example, 0.1 to 10sec) the capacitor 79 voltage exceeds the reference voltage E2. Whenthis occurs, the second comparator output goes low resetting the FF 75,Therefore, the switching means 73 is turned on and the battery 71 can bedischarged.

MOSFET 77 is also switched on discharging capacitor 79, and after a settime interval (for example, within 10 msec), the output of the secondcomparator 710 goes high.

If the over-current condition persists when the switching means 73 isturned back on, the operation described above for over-current isrepeated and the switching means 73 is again turned off cutting offbattery 71 discharge. On the other hand, if the over-current conditiondoes not persist, the switching means 73 continues in the on state andbattery discharge is possible.

FIG. 8 is a circuit diagram of the seventh embodiment of the presentinvention. In this figure, 820 is the first comparator which comparesthe measured value from the current detection means 82 with the firstreference voltage E3, 821 is the second comparator which compares themeasured value from the current detection means 82 with the secondreference voltage E4, and 822 is the first FF which is set based on theoutput of the first comparator 820. The output of this first FF 822passes through an inverter 827 to control the switching means 83 on oroff. Continuing, 823 is the second FF which is set based on the outputof the second comparator 821. 824 is the first MOSFET, 825 is the firstcurrent source, and 826 is the second MOSFET. The first MOSFET 824, thefirst current source 825, and the second MOSFET 826 are series connectedin that order and the combination is connected in parallel with thebattery 81. The first MOSFET 824 is a normally off (enhancement mode)FET which is controlled on or off by the output of the first FF 822through inverter 827. The second MOSFET 826 is a normally on (depletionmode) FET which is controlled by the second FF 823.

Further, 828 is the second current source connected in parallel with theseries combination of the first current source 825 and the second MOSFET826, 829 is a capacitor connected in parallel with the first MOSFET 824,and 830 is the third comparator which compares the capacitor 829 voltagewith the third reference voltage E5 and resets the first FF 822 and thesecond FF 823 depending on the comparison results.

In this over-current protection circuit, the first reference voltage E3is related to the second reference voltage E4 such that E3<E4. Forexample, the first comparator 820 outputs a low level signal when acurrent of 5 A or more flows, and the second comparator 821 outputs alow level signal when a current of 10 A or more flows.

When normal discharge current (less than 5 A) flows in the battery 81 ofthis over-current protection circuit, the outputs of both the firstcomparator 820 and the second comparator 821 are high level signalswhich reset both the first FF 822 and the second FF 823. This causes thefirst MOSFET 824 to be in the on state preventing capacitor 829 fromcharging. Therefore, the third comparator 830 maintains a high leveloutput, the first and second FF's 822, 823 remain in the reset state,and the switching means 83 continues to be on.

Next, if an over-current greater than 5 A but less than 10 A flows inthe battery 81, the output of the first comparator 820 goes low and thefirst FF 822 is set. This turns the first MOSFET 824 off and at the sametime turns off the switching means 83 cutting off battery 81 discharge.

At this time, since the output of the second comparator 821 remainshigh, the second MOSFET 826 remains in the on state. Consequently,capacitor 829 is rapidly charged by both the first and second currentsources 825, 828, and after the set time interval (for example, 0.1 to 5sec) the capacitor 829 voltage exceeds the third reference voltage E5.This makes the third comparator 830 output drop low to reset the firstand second FF's 822, 823 returning to the initial state. Namely, theswitching means 83 is turned on and the battery 81 is capable ofdischarging.

Elsewhere, the first MOSFET 824 also switches on discharging thecapacitor 829, and after a set time (for example 10 msec) the output ofthe third comparator 830 returns high.

In other words, when an over-current greater than 5 A but less than 10 Aflows in the battery 81, the switching means 83 shuts off and battery 81discharge is cut-off. Then after 0.1 to 5 sec, the switching means 83automatically turns on. At this point, if the over-current conditionpersists, the switching means 83 again shuts off cutting off battery 81discharge. Subsequently, the switching means 83 repeatedly turns on andoff until the over-current condition has been alleviated. However, whenthe over-current condition is removed, the switching means 83 remains inthe on state allowing battery 81 discharge.

Next, if an over-current of 10 A or more flows in the battery 81, theoutputs of both the first and second comparators 820, 821 go low andboth the first and second FF's 822, 823 are reset. This turns off boththe first and second MOSFET's 824, 826 as well as turning off theswitching means 83 to cut-off battery 81 discharge.

Since both the first and second MOSFET's 824, 826 are turned off, afterthe switching means 83 is shut off, capacitor 829 is slowly charged byonly the second current source 828. After the set time interval (forexample, 1 to 10 sec), capacitor 829 voltage becomes greater than thethird reference voltage E5. Under these conditions, the third comparator830 output goes low to reset the first and second FF's 822, 823 andreturn to the initial conditions with the switching means 83 on and thebattery 81 capable of discharging.

In other words, when an over-current 10 A or more flows in the battery81, the switching means 83 is turned off to cut-off battery 81discharge. Then after 1 to 10 sec have elapsed, the switching means 83is automatically turned on.

As described above in the seventh embodiment, after the switching means83 is turned off, depending on the battery 81 current, there are twodifferent set time intervals until the switching means 83 automaticallyreturns to the on state.

FIG. 9 is a circuit diagram of the eighth embodiment of the presentinvention. In this figure, 931 is a differential amplifier whichamplifies the voltage across the terminals of the current detectionmeans 92, and 932 is a peak-hold circuit which captures the peak valueof the differential amplifier 931 output and stores it in capacitor 933.Continuing, 934 is a discharge resistor connected in parallel withcapacitor 933, 935 is a comparator which compares the hold value of thepeak-hold circuit 932 (namely, the capacitor 933 voltage) with thereference voltage E6, 936 is a delay circuit which delays the comparitor935 output by a set interval (for example, 1 sec) before application tothe switching means 93.

When the battery 91 discharges with a normal current of less than 5 A,capacitor 933 does not charge to a voltage that exceeds the referencevoltage E6. Consequently, the comparitor 935 maintains a high leveloutput and the switching means 93 continues in the on state.

Next if an over-current of 7 A, for example, flows in the battery 91from the time labeled t0 in FIG. 10, the capacitor 933 of the peak-holdcircuit 932 charges to a voltage Cv which exceeds the reference voltageE6 after time t1, as shown by the solid line in FIG. 10. This causes thecomparator 935 output to go low. This low level signal is delayed a setinterval (namely from time t1 to t2 in FIG. 10) by the delay circuit936, and then applied to the switching means 93. This results in turningthe switching means 93 off to cut-off battery 91 discharge.

Once the switching means 93 is turned off, capacitor 933 dischargesthrough resistor 934, and by time t3 capacitor 933 voltage Cv hasdropped below the reference voltage E6. This causes comparator 935output to again go high, and again after the set time interval at timet4 (time interval t3 to t4 is equal to time interval t1 to t2) theswitching means 93 is returned to the on state.

On the other hand, if an over-current of 10 A flows in the battery 91from time t0, capacitor 933 voltage Cv (broken line in FIG. 10) exceedsthe reference voltage E6 after time t1, comparator 935 output goes low,and the switching means 93 turns off at t2 to cut-off battery 91discharge in the same manner as described previously.

Again, once the switching means 93 is turned off, capacitor 933discharges through resistor 934. However, a time t5 longer than t3 isrequired for capacitor 933 voltage Cv to drop below the referencevoltage E6. When this occurs at time t5, comparator 935 output returnsto high, and after the previously described set time interval, theswitching means 93 returns to the on state at time t6.

In other words, in this eighth embodiment, once the switching means 93is shut off due to over-current, the time interval until it returns tothe on state varies approximately in proportion to the battery 91over-current.

In the over-current protection circuits described above, when more thanthe specified current flows through the battery, the switching means isshut off by the control means. These circuits have the feature thatafter a set time interval from the switching means shut off time, theswitching means is automatically returned to the on state. Therefore,the battery is reliably protected and is returned to a state allowingsuitable discharge after the over-current discharge condition has beenrelieved.

What is claimed is:
 1. A battery over-current protection circuitcomprising:(a) a first series circuit of a current detection means fordetecting a battery current connected in series with a first switch,said first series circuit for connecting in series with the battery; (b)a second series circuit of a resistive element connected in series witha second switch, said second series circuit connected in parallel withsaid first series circuit; and (c) a control means for turning off saidfirst switch and turning on said second switch when the battery currentbecomes greater than a specified value, and for turning on said firstswitch when the battery current becomes less than the specified value;wherein said control means includes a voltage detector for detectingwhen the battery current becomes less than the specified value, andwherein said control means is further for switching said first switch onand said second switch off when said voltage detector detects that thebattery current has become less than the specified value.
 2. A batteryover-current protection circuit as recited in claim 1, wherein saidcontrol means further includes a switch control means for controlling aswitching state of said first and second switches, wherein an output ofsaid current detection means and an output of said voltage detector areinput to the switch control means, and wherein said switch control meansis for switching said first switch off and said second switch onaccording to the output from said current detection means, and whereinsaid switch control means is further for switching said first switch onand said second switch off according to the output from said voltagedetector.
 3. A battery over-current protection circuit as recited inclaim 1, wherein said voltage detector senses a voltage across theterminals of a load connected to the battery.
 4. A battery over-currentprotection circuit as recited in claim 1, wherein said current detectionmeans comprises a low valued resistive element and a voltage detectorfor detecting a voltage across said low valued resistance element.
 5. Abattery over-current protection circuit as recited in claim 4, whereinsaid first switch is a MOSFET and the internal resistance of the MOSFETis used as said low valued resistive element of said current detectionmeans.
 6. A battery over-current protection circuit comprising:(a) aswitch connected in series with the battery; (b) a current detectionmeans for detecting a value of a current flowing in the battery; (c) acontrol means for controlling an on and off state of said switchaccording to the value of the current detected by said current detectionmeans, wherein said control means is for turning off said switch whenthe value of the current is greater than a specified value and thenautomatically turning back on said switch upon expiration of a presettime interval from a time when said switch was previously turned off,said preset time interval being approximately proportional to the valueof the current.
 7. A battery over-current protection circuitcomprising:(a) a switch connected in series with the battery; (b) acurrent detection means for detecting a value of a current flowing inthe battery; (c) a control means for controlling an on and off state ofsaid switch according to the value of the current detected by saidcurrent detection means, wherein said control means is for turning offsaid switch when the value of the current is greater than a specifiedvalue and then automatically turning back on said switch upon expirationof a preset time interval from a time when said switch was previouslyturned off; wherein said control means includes a first comparator whichcompares an output voltage of said current detection means with a firstreference voltage, a flip-flop which is reset by an output of said firstcomparator, a capacitor which is discharged by an output of saidflip-flop, a current source which charges said capacitor, and a secondcomparator which compares a voltage of said capacitor with a secondreference voltage.